The invention relates to a digital circuit which receives a serial input signal and which suppresses fast signal variations.
Electronic consumer products, for example television receivers, are often controlled by means of a remote control device. Such remote control devices comprise a transmitter whose signals are received by a receiver in order to be applied to a microprocessor which executes the operation desired by the operator.
In the case of television receivers, infrared light is frequently used for transmitting such signals. The infrared receiver receives serial signals from the infrared transmitter which are applied to the microprocessor via a controllable amplifier. During prolonged transmission intervals, i.e. when no signal is transmitted by the infrared transmitter, the amplifier is controlled so that the gain is maximum. Consequently, spurious light received by the infrared light receiver can reach the microprocessor via the amplifier.
The serial signal has two states which are customarily referred to as "1" and "0", or "high" and "low". The spurious light causes fast variations in the serial signal, i.e. the serial signal assumes a state only very briefly, for example the state "1". The frequency at which the states of the serial signal change is much higher in the case of spurious light than in the case of a transmission signal.
The microprocessor evaluates the transmission signal. The appearance of such a disturbance briefly interrupts the execution of the normal program. When such interruptions due to the higher-frequency disturbances occur more often, the normal program of the microprocessor will be disturbed. From Japanese Kokai 52-129.271 a filter circuit is known which suppresses the described disturbances. In addition to several logic circuits and comparators, the filter circuit comprises a count-up/count-down counter.